A hydrotalcite is generally represented by the chemical formula of MII1-xMIIIx(OH)2(An−)x/n.(1−3x/2)H2O, wherein MII is a divalent metal cation, MIII is a trivalent metal cation, An− is an n-valent anion and x is in the range of 0.20 to 0.33 (see J. Mater. Chem., 12: 3191-3198, 2002; and J. Therm. Anal. Cal., 84: 473-478, 2006).
A hydrotalcite is generally used as an additive to improve chlorine-resistance and thermoresistance of various synthetic resin products such as polyvinyl chloride (PVC) and polyurethane, and various preparation methods of preparing thereof which use a metal salt, a metal oxide, a metal hydroxide, a urea, etc. have been disclosed (see U.S. Pat. No. 4,351,814 in the name of Kyowa Chemical Industry Co. issued on 1982 Sep. 28; U.S. Pat. No. 4,904,457 in the name of Aluminum Company of America issued on 1990 Feb. 27; U.S. Pat. No. 5,250,279 in the name of J. M. Huber Corporation issued on 1993 Oct. 5; and Korean Patent No. 454273 in the name of Doobon Co., Ltd. issued on 2004 Oct. 14). However, the hydrotalcites prepared by these methods tend to undergo undesirable physical property changes when introduced in a synthetic resin due to the presence of water molecules on the surface and between the layers. Such H2O molecules can be removed by high temperature treatment but structural deformation may also occur due to dehydroxylation (see J. Therm. Anal. Cal., 84: 473-478, 2006).
The present inventors have found that crystal water molecules of a hydrotalcite additive lowers the thermoresistance of a synthetic resin and found that a structurally stable hydrotalcite having a minimal amount of crystal water can be prepared.